Pharmacological manipulations of the extinction process of fear-induced ultrasonic vocalization in rats

The effects of pharmacological manipulation on the extinction process of fear-induced ultrasonic vocalizations (USVs), which are considered distress calls related to anxiety, were investigated. Male Wistar rats were conditioned to emit USVs by being given repeated electrical foot-shocks while in a chamber. After 10 sessions of conditioning, the animals started to emit USVs upon mere exposure to the shock chamber without being shocked. Using these animals, the extinction process of the USVs was examined. With repeated exposure to the chamber without shocks. the USVs first increased and then gradually decreased, i.e., the extinction burst was observed. Daily intraperitoneal injections of a benzodiazepine-GABA receptor agonist diazepam (DZP; 1.9 mg/kg) or a tricyclic antidepressant clomipramine (CLM; 20 mg/kg) inhibited this extinction burst. Moreover. CLM, but not DZP, shortened the period required for extinction as compared with the vehicle-treated animals. Following the extinction phase, the emission of USVs was enhanced by the cessation of both drug treatments. These results suggest that CLM would be useful for reducing anxiety-related behaviors in the extinction process. as long as withdrawal symptoms after long-term drug treatments are taken into consideration.